|
You need to understand the cure mechanism for POR-15 and other paints.
1st - steel can hold moisture in its micro-surface texture. 2nd - paints are polymers that are not totally water resistant because all polymers absorb moisture into their molecular structure. Water molecules can migrate through polymers to the surface of the steel and cause rust. Water trapped within the steel surface texture can also cause rust under the paint coating. You have all seen rust blisters that form under paint if the steel is not properly sealed.
POR-15 is a Polyurethane that cures by absorbing water molecules from the steel surface and atmosphere. It takes several hours to cure, depending upon the temperature and humidity. The higher the humidity, the faster the cure. The black color is from carbon black filler.
The thicker the applied paint coating, the slower moisture will migrate through the coating. But paint coatings also have limits on how thick they can be applied before the coating will have other problems. Most paints use solvents to thin them for spraying. The solvents must escape during the drying process, so the paint can only be so thick and a drying time between sequential coatings to allow evaporation of the solvent.
POR-15 does not have solvent in it, but the moisture cure process will create micro-bubbles if the coating is applied too thick in one layer because gas molecules are liberated during the cure process which migrate out of the paint during curing, unless it it too thick, which can trap them and allow the formation of micro-bubbles. POR-15 can be applied in multiple thinner coats to prevent the micro-bubbles. Once the coating is fully cured it will be very hard, durable and chemically resistant. Since it is so resistant to chemicals and so smooth on the surface, it is really difficult to apply any other coating over the cured POR-15 and have paint adhere to it. Some might see "fisheyes" or non-wetout areas while painting over the top of the cured polyurethane because there is nothing to cling to because of the slick smooth surface.
Most top coatings will chemically attack previously applied paint because the solvent will absorb into the prior coating and soften it and allow the top coat to gain some adhesion. This does not happen sufficiently to get good adhesion when top coating POR-15, therefore the surface of POR-15 must be mechanically scratched to allow any top coat paint to adhere mechanically by flowing into the surface scratches and locking onto them.
Epoxy primer coatings also need to have their surfaces scratched to allow paint top coats to bond to them if they are allowed to fully cure. Epoxy cures by means of a chemical cross-linking process and they are also very chemically resistant and hard.
All coatings need the steel surface to have some micro-texture to allow the paint to adhere and obtain a mechanical lock to the surface. That is why all paint manufacturers give instructions to process the surface of the steel by sandblasting or sanding to provide the roughness the paint needs to obtain proper bonding to the steel surface. A phosphate metal prep wash will also give a micro roughened surface to promote better adhesion.
The first paint layer applied to the steel, called a base coat or primer coat, depends upon your preference. The two best polymers for the first coat are definitely either epoxy or a moisture cured polyurethane. Each has its advantages and its dis-advantages. Both should be sanded or roughed-up by a scotchbrite pad before applying any other paint over the top of them. A two-part chemically cured polyurethane (not moisture cured) is not as good as epoxy or moisture cured polyurethane for bonding and protecting bare steel.
Take note that even though a paint may be labeled as a "primer" does not mean that it should be the base coat on the steel. Many paints labeled as "primers" are actually meant to be applied over a base coat and used to fill any surface roughness, with multiple sanding and recoating to achieve the desired smoothness prior to applying the final color coat. Many years ago I used "primer" that was a lacquer based paint and the steel rusted under it. This is because most of these "primer" paints are filled with talc and other fillers to make them easy to sand but which allow water molecules to easily migrate through them to the steel below if the steel is not first coated with a proper primer, like epoxy or moisture cured polyurethane.
My choice is a moisture cured polyurethane base coat. This is due to the fact that any water that is still adhered into the micro-texture of the steel will be absorbed and prevent any future rust formation. Epoxy does not do this. It is this absorption of moisture from the steel surface that allows a moisture cured polyurethane to be used directly over slightly rusty steel with excellent results. All rust has some amount of water in its surface structure, which is removed by the curing of the polyurethane and prevents it from continuing to rust after the paint is applied.
Production car painters would choose epoxy as their base coat because it can be cured faster in a heated paint booth. Time is money.
There are several manufactures of moisture cured polyurethanes. The two that I have used are POR-15 and Chassis Saver. Moisture cured polyurethane paints do not resist UV and will be broken down if exposed for long periods of time to the sun. Epoxy will also breakdown but is better at resisting UV radiation.
I use the silver colored Chassis Saver because it has aluminum flakes added instead of carbon black. When the paint is applied, the flakes lay down parallel to the surface of the steel and create a multi-layered flake barrier that is much better at blocking the migration of water molecules through the polyurethane, hence giving it a much better rating in salt water corrosion testing. Water does not migrate through aluminum but does through polymers. Chassis Saver is also much lower cost then POR-15.
If you want to apply a second coat of Chassis Saver, you need to wait until the surface has just gone past its tacky stage and doesn't have any sticky feel. But if you wait too long, you should sand the surface to insure that your second coat will adhere properly. This also applies to epoxy paints. A second coat, or a color paint coat, should be applied over the top of the Chassis Saver before it reaches full cure to avoid sanding. A second coat of Chassis Saver can be applied after about 4 hours. It does not need to cure as long if Chassis Saver is being applied over itself as long as you don't use a solvent to thin it. A solvent thinned color coat is applied about 8-12 hours later. If the color coat is applied before the polyurethane reaches full cure, then the solvent in the paint will attack the surface and allow good adhesion. If you go past 18 hours, then it is best to sand the surface before applying the next coat. Moisture cured polyurethanes can be thinned with polyurethane solvents to make them easier to spray, but not more than about 10% by volume. Just remember to spray a thinner coat if solvent is used to prevent solvent entrapment, and let it cure a little longer before applying another coat over it.
Chassis Saver sands well once cured. I have used it for the sanding primer coats prior to final painting because it flows so smooth. It is more difficult to sand than sanding primers but it sands more easily than epoxy. And I know that if I sand through to bare metal, then the next Chassis Saver coat will properly seal the steel, which won't happen if you sand through sanding primer and repaint with sanding primer over the bare steel. It is much more resistant to rock chips than sanding primers when used under color paint coatings.
I definitely recommend Chassis Saver on the underside of your car. You don't have to apply a top coat over it if it won't be in direct sun light. It can be applied using a brush, paint roller or sprayed. It flows so well that even a brush or roller applied coating will look very smooth when cured. Two coats are recommended for best protection. Be careful not to put it on too thick in the corners and edges or it will show signs of micro-bubbles. If that happens, don't worry about it. If it bubbles badly, you could always sand it down and apply another coating over the affected area.
Don't let the paint can stay open for any longer than necessary. I always apply a layer of food plastic wrap over the open can before putting the lid back on to prevent the lid from becoming "welded" in place from cured paint. I also turn the can upside down to make sure to coat the inside lid sealing surfaces to prevent any moisture from getting inside the can so that I get longer shelf life. Pour out of the can into another container what you will use in an hour's time and then reseal the original can. Your other container can stay open for an hour or more while you dip into it if painting with a brush or roller before it starts to get thick enough to make it harder to apply. Do not pour any paint from your used paint container back into the original paint can. It will cause it to harden in the can because it will have absorbed enough moisture from the air to complete the cure process.
Wear gloves because it is very difficult to remove from your skin if it has been on your skin for more than 10 minutes. It will absorb the moisture from your skin and cure fast. Wash-up with lacquer thinner. Once cured, it will not wash out of anything. It will take two weeks to wear off of your skin before you don't look like the "tin man" anymore if you don't remove it from your skin before it cures. It adheres very well to fabrics and leather, even tire rubber. Ask me how I know!
|